[swift-users] How do you use protocol types?

[Dan Loewenherz]

Unless you need to abstract what the Factory class does, I would eliminate
AProto.

class Factory {

    func constructInstance<T: SimpleConstructableElement>(_ t: T.Type) -> T
{

        return t.init()

    }

}


If you truly want to genericize Factory, you can do the following:


class Factory<T: SimpleConstructableElement>: AProto {

    typealias ElementType = T


    func constructInstance(_ t: T.Type) -> T {

        return t.init()

    }

}


However, ElementType can be inferred, so you can zap it, and then you get
this:


class Factory<T: SimpleConstructableElement>: AProto {

    func constructInstance(_ t: T.Type) -> T {

        return t.init()

    }

}


…which compiles and I think does what you want it to. :)


Dan

On Wed, Jul 13, 2016 at 8:58 PM, Karl via swift-users <swift-users at swift.org
> wrote:

> So I’m trying to work generically to construct things based on their
> conformance to a protocol with an initialiser. I’m sure this worked before;
> I’ve done it before. Maybe there have been some changes in the language or
> something, because I’ve tried everywhich-way and this just isn’t flying:
>
> ==================
> protocol AProto {
>
>         associatedtype ElementType
>         func constructInstance(_: ElementType.Type) -> ElementType
> }
>
> protocol SimpleConstructableElement {
>         init()
> }
>
> extension Int : SimpleConstructableElement {}
>
> class Factory : AProto {
>
>         typealias ElementType = SimpleConstructableElement
>
>         func constructInstance(_ t: SimpleConstructableElement.Type) ->
> SimpleConstructableElement {
>                 return t.init()
>         }
> }
>
> Factory().constructInstance(Int.self)
> =================================
>
> /tmp/MyPlayground.playground:15:7: Type 'Factory' does not conform to
> protocol 'AProto'
> /tmp/MyPlayground.playground:6:7: Protocol requires function
> 'constructInstance' with type '(ElementType.Protocol) -> ElementType'
> /tmp/MyPlayground.playground:19:7: Candidate has non-matching type
> '(SimpleConstructableElement.Type) -> SimpleConstructableElement’
>
> =================================
>
>
> The thing that gets me about this is that Xcode’s code-completion tells me
> that SimpleConstructableElement.Type is of type
> SimpleConstructableElement.Protocol, so it should be the same as
> ElementType.Protocol and should be accepted, right?
>
> Karl
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>
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